Electrical communication



Aug. 1, 1944'.

vD. A. GRIFFIN ELECTRICAL COMMUNICATION Filed March 11, 1941 2 Sheets-511661'. 1

Aug. l, 1944. D. A. GRIFFIN 2,354,749 I ELECTRICAL `CMMUNICATION I I Filed March 11, 1941 2v sheets-sheet;

Patented Aug. 1, 1944 UNiTEof STATES PATENT o1-E|c-Ej ELECTRICAL coivnviUNic-A'rioN Dana. A. Griffin," Plainfield, N. J., assigner to Communication Measurements Laboratory,

New York, N. Y., a partnership Application March 1 1, 1941, Serial No. 382,695

18 Claims. (Cl. Z50-20) To provide for an extremely high degree of frequency selection; v

To provide improved frequency selection, in which the selectivity characteristic curve is substantially rectangular, rather than of the usual resonance curve shape;

To provide band pass selectivity, With a substantially rectangular characteristic curve, in which the width of the band selected may be easily and simply controlled and varied as desired;

To provide apparatus which may be used for the reception of either radio, telegraph or telephone signals with a substantially rectangular characteristic curve, in which the Width of the received band may be continuously and smoothly varied from zero up to 10,000 cycles or more; and in which said variation may be made at an audible rate;

To provide radio receiving apparatus for the reception of continuous Wave telegraph signals free from many of the difficulties characterizing even the best of such present-day apparatus;

To provide such apparatus giving an improved signal-noise ratio, and a minimum of receiver noise during the intercharacter spaces;

To provide such apparatus enabling the operator to receive telegraph signals with a pleasing tone, easy to copy, and which is not a function of the receiver tuning, and which is not subject to change in pitch with drifting of the frequency of the beat frequency oscillator;

To provide such apparatus in which the audio note of the telegraph signal is produced by frequency modulation of the conversion oscillator;

To provide such apparatus in which the frequency deviation of the conversion oscillator is controlled automatically by the amplitude of the signal being received;

Still other objects and advantages of my invention will be apparent from the specification.

In this application I have particularly pointed out and distinctly claimed the part,'improvement or combination which I claim as my invention or discovery and I have explained the principles thereof and the best mode in which. I have contemplated applyingthose principles so as to disl tinguish my invention from other inventions.

In the drawings:

Figure l is a simplified diagram illustrating the principles of operation of apparatus according to my invention;

Figure 1a is a simplified diagram of a modifi cation of my invention, particularly useful in the..-

reception of continuous Wave telegraphy; v

Figs. 2, 3, 4, 5, 6 and? are curvesV explaining theoperationof my invention, and Y Fig. '8 is a circuitdiagram showing the circuit I according to one form. of my invention.

In accordance with the best conventional' prac-lv tice, vcontinuous Wave telegraph" signals are received in a superheterodyne receiver using a beat frequency oscillator to beat with the signal and'. give an audible tone, vand selectivity is obtained by using a crystal filter circuit in the intermedi` ate frequency amplifier.

This type of receiver has several defects. It

is critical in adjustment, drift of the frequency of the beat frequency oscillator causes a change in the pitch of the signal. note, and it is difcult to hold the signal note in the range ofV pitch which is easiest to copy. Furthermore, if the;

crystal is used in its most selective adjustment, the set noises, incombination With the beat frequency oscillator, cause a continuous ringing noise which makes weak signals hard to copy, and with fast keying, the signal tones slur into the spaces, because of the high Qv of the crystal circuit. Y l

For telephone reception, the crystal iilter is not well suited, because the nose of its resonance curve is too sharp, and the skirts are too broad.

`In accordance with my invention, I eliminate i these difficulties by eliminating the crystal filter and the beat frequency oscillator which produces audio beats, and by employing, instead, in my preferred form, a triple detection system, employing a first conversion. oscillator and detector to produce a first intermediate frequency, a sec- `I obtain frequency selectivity by the use of.. *infinite rejection circuits in the intermediate frequency stages, these infinite rejection circuits and after the second detector. This will be mostA easily understood by reference to Fig. 1.

Infinite rejection circuits are well known in the art, and in general, it may be stated that they employ compound coupling, with the inductive coupling arranged t oppose and neutralize the capacitive coupling for one particular fre- For the purpose of attenuating the frequencies which are transmitted by the first circuit, the output of the high I. F. amplifier may be impressed upon a second high cut-off infinite rejection circuit, like the first infinite rejection circuit. The action of this second circuit is the same as that of the first and it serves to attenuate still further the frequencies attenuated by the first circuit.

Thus any intermediate frequency higher than 465 kilocycles will be severely attenuated, because for any frequency above that value the attenuation is practically innite and remains so. For

example, an incoming signal of 990 kilocycles will be practically excluded, while an incoming signal of 1001 kilocycles will be admitted.

quency, so that at that particular frequency the attenuation of the circuit is practically innite. For a description of such circuits and their characteristics, reference may be had to QST for November, 1937, pp. 19 et seq. Y

Such circuits may have a frequency attenuation characteristic such as shown in Figs. 2 and 3, wherein the ordinates represent attenuation and the'abscissae frequency. The curve of Fig. 2isthat of what I term a low pass, or high cutoif circuit, while that of Fig. 3 is what I term a high pass, or low cut-off circuit. The difference is that in the low pass circuit, the attenuation gradually decreases with increase in frequency until it reaches the critical frequency, at which the attenuation rises extremely rapidly to practically infinity, and remains at a high value for` further increase, while the high-pass characteristie is substantially the mirrorV image of the low-,pass characteristic.

I prefer to employ a pair of cascaded low pass or high cut-off circuits to determine the upper frequency limit of the band to be selected, and a pair of low cut-off circuits in cascade to determine the lower frequency limit of the selected band. The band of frequencies included between the upper and lower limits or width of the selected band may be controlled by the tuning of the second conversion oscillator solely, and does not require any adjustment of the high or lowpass circuits. Why this is the case will be explained hereafter.

Referring now more particularly to Fig. 1, in-

coming signals from the antenna may be impressed upon the first detector system supplied with heterodyning oscillations from the first oscillator. The rst detector system may comprise one or more stages of amplification at received signal frequency, and one or more circuits tuned to signal frequency, as well known in the art. For the purpose of explaining the operation of my invention, it may be assumed that the rst intermediate frequency is 465 kilocycles, the incoming signal is 1000 kilocycles, and the first oscillator frequency to tune this signal is 1465 kilocycles.

The output of the first detector system may be supplied to the first infinite rejector system, which 'ina'yjliave-its critical or infinite vattenuation "freque'lcy'set at' 465 kilocycles or slightly above, i. e. fromv a fraction of a kilocycle to two or threekilocycles, as'the case ,may be. Under these conditionsany frequency withina few kilo' l cycles of 465, but lower 'than I465, will pass through the first infinite ,rejection vcircuit andbe impressed upon the high intermediate frequeny,AV

amplier system.

The output of the second infinite rejection circuit may be impressed upon the second detector system, supplied with local oscillations from the 'second conversion oscillator, which, for telephone reception will not be modulated, but for telegraph reception of continuous wave signals may be frequency modulated by the modulator system, supplied with the desired audio frequency tone by an audio frequency oscillator. Switch S may be closed when telegraph reception is desired, and left open for telephone reception.

A portion of the output of the second infinite rejection circuit may be rectified and impressed upon the frequency modulator, for telegraph reception, in a manner which will be described later. This, however, does not affect the principles of the circuit as a whole, and need not be considered further at this point.

The frequency of the second conversion oscillator may be chosen of a value to reduce the signal frequency still further, while still maintaining it above audibility. Purely by way of example again, it may be assumed to be 400 kilocycles, giving an out-put, from the second detector of kilocycles. The output of the second detector may be supplied to the third infinite rejector circuit, arranged to provide a low side cut-off at 65 kilocycles, i. e., tuned to a frequency from a fraction of a kilocycle to several kilocycles below 65 kc. Any frequency less than 65 kilocycles will be greately attenuated, while frequencies a few kilocycles above 65 will be passed through to the 10W intermediate frequency amplifier.

The output of the low intermediate frequency amplifier may be fed to a fourth infinite rejecticn circuit having a low side cut-olf, similar to the third circuit, and tuned to a frequency from a fraction of a kilocycle to-several kilocycles below that to which the third infinite rejector circuit is tuned, and serving to still attenuate the undesired frequencies which may pass through the third innite rejector circuit. The output of the fourth circuit may be supplied to the third detector system, the output of which may be supplied to any suitable audio frequency amplifier system and signall indicator.

The effect of varying the frequency of the second conversion oscillator may now be seen. With the frequencies assumed, a signal of 1,000 kilocycles is the only onel that can come through to the third detector. Any signal frequency less than 1000 kilocycles will be cut off by the high side cut-off circuits, whiletfany signal greater than 1000 kilocycles will be cut off by the low cut-olf circuits.. This condition is shown by the over-all selectivity curve of Fig. 4.

If the second conversion oscillator be now 'tuned above 400 kilocycles, all frequencies which pass the high side cut-off circuits, after the second detection, will be too low to pass through the low side cut-off circuits, and nothing will be received. This may be visualized by assuming the two curves which make up the upper and lower frequency limits in Fig. 4 to be moved t0- ward each other until they pass, and the space between them, which represents the frequency Width of the admitted band, becomes zero.

On the other hand, if the conversion oscillator be tuned to a frequency below 400 kilocycles, the effect is just the reverse, the curves of Fig. 6 being in effect moved further apart, widening the over-all admittance band and producing a response characteristic such as shown in Fig. 7,

If the high and low side cut-off circuits be interchanged, then to expand the band it would be necessary to tune the conversion oscillator above 400 kilocycles.

In any event, it will be seen that by varying the frequency of the second conversion oscillator, and without changing the tuning of the cut-off circuits the admittance band may be broadened or narrowed as may be desired, and this change in width does not take place in discrete steps, but is a continuous and smooth function of the second oscillator frequency. This lends itself to optimum reception under widely varying conditions. For instance, if telegraph reception is desired under severe interference conditions, the second conversion oscillator may be tuned so as to establish an extremely narrow admittance band. On the other hand, if high fidelity telephone reception is desired, the second conversion oscillator may be set to provide an admittance band of 10,000 cycles, or even more. Should in terference develop, the band may be narrowed to just the extent desired to provide the best compromise between interference and quality.

At this point, it may be noted that variation of the tuning of the second conversion oscillator to change the band width may require a slight retuning of the iirst oscillator to keep the carrier frequency of the incoming signal after conversion, in the center of the over-all admittance band of the receiver. This is because varying the tuning of the second conversion oscillator does not in effect change the frequency at which both high and low side cut-offs occur, but only the low side. That is to say, referring to Fig. changing the conversion oscillator frequency may be regarded as moving the low frequency cuteoff toward or away from the high frequency cut-off, which remains unchanged; hence a signal which has its carrier frequency centered in the admittance band for one band width, may need to be re-centered by a slight readjustment of the first oscillator frequency in case the width of the admittance band is changed substantially.

Referring now more particularly to Fig. 8, I have shown the details of the circuit of a preferred form of my invention, with the exception of the circuits ahead of the output of the first detector, which is well-known in the art and forms per se no part of my invention. The output of the rst detector system is fed into the rst infinite rejector circuit I0 which may comprise coupled inductances Illa, and Illb,` Va third inductance Ic, capacities IIld, Iile and IIlf, and variable resistance IUg. Since these circuits are,

per se, well-known in the art, and the method of establishing the values of the various constants for particular frequencies are also known,

the same is not believed necessary to be discussed. Y

The output of the first infinite rejector system may be impressed upon the control grid I'Ib of the amplifier tube II, having a cathode I Ia, control grid IIb, screen grid IIc and anode IId. Other types of tubes may be employed as will be understood, the tubes herein shown and described being merely by way -of example. The output of tube II may be fed to the second innite rejection circuit I2 which again may comprise coupled inductances I2a and |217, third inductance |2c, capacities |211, I2e and |2f and variable resistance |2g, and may be tuned to a frequency slightly higher than that of first infinite rejector circuit IB. The output of circuit I2 may be supplied to the control electrode of the second detector tube I3, which may have cathode I3a, control electrode |311, screen electrode |30 and I3d, injector electrode |3e and anode |31.

The output of the tube I3 may be supplied vto the third infinite rejector circuit (low ,side cute' off) I Il, which may comprise coupled inductances Ida and I4b, third inductance Ic, capacities Idd, |4e and |4f and variable resistance |49. The out-` put of this circuit maybe supplied to the low I. F. amplifier tube I5 which may comprise cathode |5a, control grid |517, screen grid |50 and Yanode |5d. Each of thetubes II, I3 and I5 may be provided with a resistance in a cathode lead to ground, these being IIg, I 3g and I5g, shunted by capacities IIh, I 3h and |5h respectively.

The output of tube I5 may be supplied to the fourth infinite rejection circuit I6 which may be a low side cut-off circuit,l preferably tuned to a frequency slightly lower than that of thethird infinite rejector circuit I4. The fourth infinite rejection circuit may comprise inductances Ilia, and I6b coupled together, third inductance Ic and capacities Id, IEe and ISf and variable resistance |6g.

The output of the fourth innite rejection circuit may be supplied to the anode I 'Ib of diode I1, thecathode Ila of which may be connected to ground. One side of inductance Ib may be connected through resistance 30 to ground, said resistance may be shunted by capacity 3|, thel audio frequency output being taken through 'connection 32 as shown. Since audio frequency amplier and indicator circuits are well-known in the art and form per se no part of my invention, they are not described `in detail. I x

The oscillation to be applied to tube I3 for the purpose of reducing the signal from the high intermediate frequency to the low intermediate frequency may be generated in tube I8 which may have cathode Illa, control electrode ISb and anode I8c. This tube may be provided with a feedback or oscillator circuit of any usual type'such, forinstance, as that shown, having inductance Illa and condenser |90 connected between ground and control electrode I8b, a connection from cathode I8a through resistance IBg, shuntedby condenser Ih, to ground, and the output circuit connected from anode |8c through inductance I9?) which may be coupled to inductance 4I9a, ,thence to ground through capacity IQd and to the source'of plate voltage through radio frequency choke RFC. l

The anode yIc may also be connected through capacity ISe to the injector grid I3e of the second detector, which may be connected to ground,- through radio frequency choke RFC. Selection of the frequency of the conversion oscillator I3 may bemade by varyingv condenser |9c or in any other 'suitable manner known in the art. With switch 2l) in' open position as shown, the tube la'operatesas a simple conversion oscillator supplying unmodulated oscillations to the tube I3 and the vappa-ratus may beused in this condition for the reception of telephone signalsor amplitude modulated Waves. It is not suitable for the reception of continuous wave signals having a constantcarrier frequency. For the reception of such signals, switch 2B may be closed to frequency modulate oscillator I8. Switch 2G may connect the control grid I8b of oscillator I8 through resistance 26 to grid 24h of frequency modulator'tube 24. This tube may have cathode-24a, grid 24h, control grid 24d, screen grids 24e and 24o, and anode 24j connected through condenser 34 to the upper terminal of resistance 26. Anode 24 may also be connected through radio frequency-.choke RFC to the source of plate voltage. Cathode 24a. maybe connected through resistance 24g to ground, the resistance being shunted by condenser 24:71.. Grid 24d may be connected through resistance and capacity 3i)V to ground. Capacity 36 may be shuntedby additional resistance-29.

Any suitable variable audio frequency oscillator 8 may be provided for establishing the note or tone to be impressed upon continuous wave signals. One terminal of this oscillator may be connected to ground, and the other terminal to variable connection 9 on resistance 28. The tube 24 and its associated circuit forms a frequency modulator (in effect a variable inductance) serving to deviate the frequency generated by the conversion oscillator I8 and since such circuits are per se well known in theV art and form no part of this invention, the same are not described in detail. It may be noted, however, that setting the frequency of the audio oscillator 8 fixes the-tone introduced into the circuit, whereas adjustment of connection 9 controls the magnitude of the deviation of the frequency of oscillator 8.

For an understanding of the operation of the circuit in receiving continuous wa-vetelegraph signals in one mode of operation, reference may be had to Fig. 6. Referring t Fig. 6, the curve may represent the over-all admittance band of the system as it appears at the output of the fourth innite rejector circuit, the conversion oscillator I8 being tuned to such afrequency that the admittance band of the receiver as seen at this point extends from 64 tov 66 kilocycles, and theresultant or beat frequency output between the incoming signal and the local oscillation having a carrier frequency of 64 kilocycles for example just at the lower edge of the admittance band and being deviated in frequency by the frequency modulation impressed upon the local oscillation from 62.5 kilocycles to 65.5 kilocycles.v For this, it is assumed that the amplitude of both the incoming signals and the local oscillation is constant so that no amplitude modulation effects appear up to this point, the only modulation being the frequency deviation applied to the local oscillator.

If the deviationimpressed by the frequency modulator were less than 2 kilocycles, and this deviation occurred entirely within the admittance band, nothing would be heard with the possible exception of key clicks or mush If the deviation is as shown .in Fig.. 6, so that the frequencies swing into and out of the admittance band, this change occurring once per cycle, itwill be' seen that an amplitude modulation effect is produced, the same being a complete passage of signals or no passage of signals at al1, and since the signal passesV into and out of the cut-off, zone once fori each deviation cycle and the deviation frequency is fixed, for example, at 256 cycles per second, it is apparent Vthat a 256 cycle note will be introduced and will be reproduced by the telephone or loud speaker, this note continuing while the signal oscillations are being received and 0f course being silent during the intercharacter spaces.

In the example above, the mean carrier fre- 4 quency was given as at one edge of the admittance band, i. e., 64 kc. This is not necessary, however, and the mean -carrier frequency may be either within or Without the band, so long as it is deviated into and out of the band once per cycle.

For minimum noise on inter-character spacing, using the apparatus in this manner, it may be preferable to adjust the mean frequency of the second conversion oscillator luntil the admittance band is less than Zero, that is to say, until the curves of Fig. 4 have been in effect pushed together until they have passed, as explained supra, and the receiver has no admittance band. Then deviating the frequency of the second conversion oscillator at an audio rate, as by frequency modulation, in effect widens and narrows the admittance band at an audio rate, and control of the magnitude of the frequency deviation controls the magnitude of the maximum width which the admittancev band may reach and introduces the audio tone.

Under certain conditions, it is possible to obtain a double frequency tone, that is a 512 cycle note with the audio oscillator set for 256 cycles per second. To do this, it is necessary to 13e-tune the fourth infinite rejection circuit so that it is a high-side cut-off circuit, the third infinite rejection circuit being left as before, but the circuit i6 being readjusted to provide high side cut-off slightly above thel cut-off frequency of the loW side cut-off circuit I4 for example at 66 kc. The admittance band width of the low I. F. channel may be from 64 to 66 kc. as shown by Fig. 7, and in this case the band width of the high I. F. channel is not critical, because the low I. F. channel now provides both high and low side cut-off.

If now the first oscillator be slightly re-adjusted, so as to move the carrier frequency from 64 kilocycles to 65 kilocycles, a curiousefect will be noted. The frequency deviation will now be from 63.5 kilocycles to 66.5 kilocycles, and the deviation will cause the signal frequency to passinto and out. of the admittance band of the low I. F. channel, both on the higher and lower sides twice per cycle instead of once. The portion of the frequency distribution which is outside of the admittance band both in Figs. 6 and 7 has for conveniencey been shaded and it will be seen readily by comparing Figs. 6 and 7 that a double frequency component is introduced under the condition of Fig. 7; that is to say, instead of the tone from the loud speaker being 256 cycles, it will be 512 cycles per second. If now either conversion oscillator be slightly retuned to shift the mean carrier frequency either way so that the frequency does not pass out of the admittance bari-:I on both sides but only one side, the tone will drop back tothe fundamental, i. e. 256 cycles, in the example here given.

In the example just given, for obtaining the double tone frequency, the mean carrier frequencyhas been within the admittance band, as shown in Fig. 7, but the same effect may be produced by Yadjusting the mean carrier frequency to beout of the admittance band, and deviating it through the admittance band and beyond it.

. The double tone frequency gives a new'kind of selectivity which is of advantagel wherel the desired signal is interfered with by signals having frequencies near that of the desired signal, and lying on both sides of it. In this case, it is possible to deviate the desired signal frequency to give the double tone, while the'signals lying on either side of it pass into and out of the admittance band only once per cycle. Then the interfering signals will have a tone of 256, for example, while the desiredsignal has a tone of 512, and the desired signal may be easily read.

In the foregoing we have described deviating the frequency of the local oscillator I8 so as to throw the resultant frequencies within and without the over-all admittance band. It will be understood, however, that a similar result can be obtained by holding the frequency of the local oscillator ls constant and deviating the cut-off frequency of theifourth or third infinite rejector circuit or both. This has been shown in- Fig. 8. For example, we mayI provide switch ZSinterposed in a connection from the top of resistance 26 to the top point of inductance lGb in the fourth infinite rejector circuit I6. To operate by deviating the frequency of this rejector circuit, the switch will be opened so that the frequency modulator tube 24 has no effect on the oscillator I8 and the switch 25 will be closed to connect this tube to the fourth rejector circuit i6. Since this frequency modulator tube operates in effect like an inductance of varying value, when the switch 2|) is open and switch 25 is closed, the effective inductance of circuit I 6 is varied and this variation produces the same effect as' previously described, the signal being alternately passed through and alternately tuned out. That is to say, it converts the beat oscillation toV a fully `amplitude-modulated oscillation having a tone set by the audio frequency oscillator.

An improved signal to background noise ratio may be obtained when frequency modulating the oscillator I8, by closing switch 2|', thereby establishing a circuit from the anode 22h of diode 22' through cathode 22a, and switch 2| to the lower end of resistance 28. This permits the diode 22 to build up a positive bias across the time constant circuit resistance and condenser 3U. During the reception of signal impulses this positive bias applied to the grid 24d of. the frequency modulator tube 24 tends to increase the amount i In connection with the adjustment of the magnitude of the deviation as controlled by connection 9, an adjustment should be made so that the minimum deviation is provided which is necessary to properly modulate the signal. Excess deviation above this amount has the effect of widening the band and is certain to increase the noise background and may even bring in an interfering signal.

It is also to be noted that the adjustments of the conversion oscillator lll-r together with the proper adjustment of the deviation control 9 do nothave tobe changed once they are properly made (unless it is desired to vary the band width as above described). Signals may be tuned in and out merely by operating the main tuning control of the receiver, in this instance the tuning control of the first oscillator.

While I have shown and described certain preferred embodiments of my invention, it will be understood that modifications and changes may be made without departing from 'the spirit and scope thereof as :will be clear to those skilled in the art.

I claim:

1'.-In a superheterodyne receiver of the triple detection type a. first conversion oscillator, a first intermediate frequency amplifier including means for attenuating. only frequencies higher 4than a predetermined frequency, a second conversion oscillator, a second amplifier operating at a different intermediate frequency including means for attenuating only frequencies lower than a predetermined frequency, and means for adj-usting theeifective signal band width of the signal produced at the output of the second intermediate frequency amplifier, said means comprising manual means for adjusting the frequency of said second conversion oscillator.

Z. In a superheterodyne receiver of the triple detection type, a rst conversion oscillator, a rst intermediate frequency amplifier including means for attenuating only frequencies lower than a predetermined frequency, a second conversion oscillator, a second amplifier operating at a different intermediate frequency including means for attenuating only frequencies higher than a predetermined frequency, and Vmeans for adjusting the effective band width of the signal produced at the output of the second intermediate frequency amplifier, said means comprising manual means for adjusting the frequency of said conversion oscillator. v

3. The method of controlling the signal admission band width for high frequency oscillations in a receiver having an intermediate frequency selecting system, which comprises heterodyning the oscillationsto produce oscillations of a first intermediate frequency, suppressing oscillations of a frequency adjacent to the rst intermediate frequency and lying on one side only thereof, heterodyning the resultant oscillations to produce oscillations of a second intermediate frequency, suppressing oscillations of a frequency adjacent to said second intermediate frequency and lying on the other side only of said intermediate frequency, detecting the resultant oscillations, and controlling the frequency of the local oscillations in the second heterodyning step to regulate the effective band width ofthe signal produced at the output of the second intermediate frequency suppression step.

4. The method of controlling the admittance band Width of a high frequency receiver and the positionv of a received carrier Wave in the ad`' mittance band of the receiver, which comprises heterodyning the oscillations to produce oscillations of a first intermediate frequency, suppressing oscillations of a frequencyadjacent to the first intermediate frequency and lying on one side thereofy heterodyning the resultant oscillations to produce oscillations of a second intermediate frequency, suppressing oscillations of a frequency adjacent to said second intermediate frequency and-'lying on the other side of said intermediate frequency, detecting the resultant oscillations, controlling the frequency of the local oscillation the second heterodyning step to regulate the bandv width of the admitted signal, and regulating the frequency of the first heterodyning step to control the position of the signal carrier Wave in the over-all admittance band.

5. High frequency selecting apparatus, comprising, in combination, means for heterodyning oscillations to produce oscillations of a first intermediate frequency, means for suppressing oscillations having a frequency adjacent to and lying on one side only of the intermediate frequency, means for heterodyning the resultant oscillations to produce oscillations ofa second intermediate frequency, means for suppressing oscillations having a frequency adjacent to said second intermediate frequency and lying on the other side only thereof, means for detecting the resultant oscillations, and means for varying the effective band width of signal produced at the output of the second suppressing means, said means comprising means for varying the frequency of the local oscillator in the second heterodyne.

6. High frequency 'selecting apparatus, comprising, in combination, means for heterodyning oscillations to produce oscillations of a first intermediate frequency, means for suppressing oscillations having a frequency adjacent to and lying on one side only of said intermediate frequency, means for heterodyning the resultant oscillations to produce oscillations of a second intermediate frequency, means for suppressing oscillations having a frequency adjacent to rsaid second intermediate frequency and lying on the other side only thereof, means for detecting the resultant oscillations, and means for smoothly and continuously varying the over-all admittance band Width of the signal produced at the output of the second intermediate frequency suppressing means, said means comprising manual means for continuously varying the frequency of the local oscillation in the second heterodyne. y

7. In a radio receiver for receiving continuous Wave telegraph signals of constant frequency, in combination, a local oscillator for heterodyning incoming signals, a frequency selective intermediate frequencycircuit, a tone frequency generator, means for deviating the frequency of the local oscillator at the frequency of said tone generator to produce frequency modulated intermediate frequency signals, means for converting said frequency modulated signals to amplitude modulated signals, and means for detecting said amplitude modulated signals.

8. The method of receiving a continuous Wave signal having a fixed carrier frequency, which comprises, selecting the desired incoming signal, frequency modulating said selected signal with an audio-frequency tone, sharply attenuating frequencies of said frequency modulated selected signal lying on one side of a predetermined value to produce amplitude modulation on said signal, and detecting the amplitude modulated signal so produced.

9. In a superheterodyne receiver, in combination, a first conversion oscillator, an intermediate frequency circuit having a sharply defined frequency cut-off on one side only of its response curve, a second frequency adjustable conversion oscillator, a second intermediate frequency cir-` cuit having a sharply defined frequency cut-off on the other side only of its response curve, and means for adjusting the effective band Width ofthe signal produced at the output of the second intermediate frequency circuit, said means comprising manual means for adjusting thte frequency of said second conversion oscilla or.

10. The combination claimed in claim 9, With means for frequency modulating said second conversion oscillator.

11. The combination claimed inv claim 9, with means for frequency modulating said second conversion oscillator at an audio rate.

12. The combination claimed in claim 9, with means for frequency modulating said second conversion oscillator at an audio rate, and means associated with said mst-mentioned interme diate frequency circuit for controlling the amount of said frequency modulation in accordance with received signal impulses.

13. In a receiver for receiving continuous wave signals having a fixed carrier frequency, in combination, a frequency selective circuit, means for passing received signals through said selective circuit, electronic means for altering the tuning of said selective circuit at an audio rate, whereby audio amplitude modulation of continuous Wave signals is produced, and means for detecting the amplitude modulated signals so produced. Y

14. Ihe method ofreceiving continuous wave telegraph signalsr of fixed frequency in a path having a variable signal admission band Width characteristic, which comprises collecting incoming energy, selecting that portion of the co1- lected energy which falls Within a predetermined frequency bandwidth and continuously varying the band Width periodically from zero through intervening values to a predetermined value. I

15. The method recited in claim 14, in which said signal admission. band Width is varied at an audible rate.

16. The method of receiving continuous wave telegraph signals having a xed frequency which comprises collecting incoming signal energy, periodically deviating the frequency thereof, said deviation being of such value as to introduce a component of double the deviation frequency, selecting portions thereof which lie between predetermined frequency limits to produce amplitude modulation thereof, and detecting the amplitude modulated signal so produced.

17. In a superheterodyne receiver, in combination, a first conversion oscillator, a rst intermediate frequencyselective circuit, a second conversion oscillator, a second intermediate frequency circuit having a high side cut-off circuit and a low side cut-off circuit in tandem, and means for deviating the frequency of said second conversion oscillator.

Y18. In al receiver for receiving continuous Wave telegraph signals having a fixed carrier frequency, means for selecting signals to be received; means for heterodyning said signals to produce signals of a different frequency; a selecting circuit through which said heterodyned signals are passed, said selecting circuit having characteristics such as to pass signals of a finite band Width and means for varying the band Width of the signal produced at the output of said selective circuit periodically at an audiblev rate.

DANA A. GRIFFIN. 

